When in the Admin panel, click the Layers tab.
Click they layer you'd like to make transparent and you'll be brought to the layer editing screen.
Scroll down to on the left-hand side and you'll see Polygon, Line, and Point as options. Click the corresponding feature type.
From here you can click the colors for both strokes and fill. Once you've clicked one of these two options, use the second sliding color bar to determine opacity.
Additionally, going into Layer Editor mode will allow you to change the layer's color. To do so, open the layer in question in the Layers tab of your Admin panel. When you've entered Layer Editor Mode, click the Stylizing button and then clicking the color you'd like to change.